Nuclear plant operators were frantically trying to keeptemperatures down in a series of nuclear reactors - including onewhere officials feared a partial meltdown could be happening Sunday- to prevent the disaster from growing worse.

Chief Cabinet Secretary Yukio Edano also said Sunday that ahydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichinuclear complex, the latest reactor to face a possible meltdown.That follows a blast the day before in the power plant's Unit 1,and operators attempted to prevent a meltdown there by injectingsea water into it.

"At the risk of raising further public concern, we cannot ruleout the possibility of an explosion," Edano said. "If there is anexplosion, however, there would be no significant impact on humanhealth."

More than 170,000 people had been evacuated as a precaution,though Edano said the radioactivity released into the environmentso far was so small it didn't pose any health threats.

"First I was worried about the quake," Kenji Koshiba, aconstruction worker who lives near the plant. "Now I'm worriedabout radiation." He spoke at an emergency center in Koriyama townnear the power plant in Fukushima.

The French Embassy urged its citizens Sunday to leave the areaaround Tokyo - 170 miles from Fukushima Dai-ichi -in case the crisis deepened and a "radioactive plume" headed forthe area around the capital. The statement acknowledged that thepossibility was looking unlikely.

Edano said none of the Fukushima Dai-ichi reactors was near thepoint of complete meltdown, and he was confident of escaping theworst scenarios.

A complete meltdown - the collapse of a power plant's ability tokeep temperatures under control - could release uranium anddangerous contaminants into the environment and pose major,widespread health risks.

Up to 160 people, including 60 elderly patients and medicalstaff who had been waiting for evacuation in the nearby town ofFutabe, and 100 others evacuating by bus, might have been exposedto radiation, said Ryo Miyake, a spokesman from Japan's nuclearagency. The severity of their exposure, or if it had reacheddangerous levels, was not clear. They were being taken tohospitals.